Tip clearance measurement system

ABSTRACT

A tip clearance measurement system (TCMS) includes a probe and sensing circuitry. The probe directs microwave signals toward a turbine blade and receives microwave signals reflected by the turbine blade. The sensing circuitry includes a switch having a first state in which a main frequency is provided at an output of the switch and a second state in which a reference frequency is provided at an output of the switch. The sensing circuitry further includes a first conditioning circuit that receives a frequency provided at the output of the switch and provides a conditioned frequency to the probe and a second conditioning circuit that receives both the conditioned frequency provided by the first conditioning circuit and a reflected frequency received by the probe, and provides a conditioned output based on the conditioned frequency and reflected frequency.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.14/185,061, entitled “TIP CLEARANCE MEASUREMENT SYSTEM” and filed onFeb. 20, 2014 which in turn claims the benefit of U.S. ProvisionalApplication No. 61/888,961 filed Oct. 9, 2013 for “TIP CLEARANCEMEASUREMENT SYSTEM” by Bhupindar Singh and Gary M. McBrien.

BACKGROUND

The present invention is related to tip clearance measurement systems,and in particular to radio frequency circuitry employed in tip clearancemeasurement systems.

Tip clearance measurement systems are utilized to measure the turbineblade tip clearance in aircraft engines. Tip clearance refers to adistance between rotating components (such as turbine and/or compressorblades) and stationary components (such as a turbine case). Efficiencyof aircraft engines is increased by minimizing the distance betweenthese rotating and stationary components. In some cases, the distancecan be controlled by selectively heating/cooling the stationary case.The case is expanded/contracted based on the heating/cooling provided,and the distance is minimized.

Tip clearance measurement systems provide feedback regarding thedistance between rotating and stationary components. A typical tipclearance measurement system uses a probe to direct electromagnetic (EM)waves toward the turbine blade. In addition, the probe monitors thereflection of the EM waves (as they reflect off of the turbine blades)and uses the monitored reflections to determine the distance between theblade tips and the stationary case. The EM waves are generated atspecific frequencies (e.g., microwave, radio frequencies). At least twodifference frequencies are required, including a reference frequency anda main frequency. Typically, separate circuits are required to conditionand provide each of these frequencies to the probe. In addition,separate circuits are required for each frequency to analyze thereflected signals received from the probe.

SUMMARY

A tip clearance measurement system (TCMS) includes a probe and sensingcircuitry. The probe directs microwave signals toward a turbine bladeand receives microwave signals reflected by the turbine blade. Thesensing circuitry includes a switch having a first state in which a mainfrequency is provided at an output of the switch and a second state inwhich a reference frequency is provided at an output of the switch. Thesensing circuitry further includes a first conditioning circuit thatreceives a frequency provided at the output of the switch and provides aconditioned frequency to the probe and a second conditioning circuitthat receives both the conditioned frequency provided by the firstconditioning circuit and a reflected frequency received by the probe,and provides a conditioned output based on the conditioned frequency andreflected frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a tip clearance measurement system according toan embodiment of the present invention.

FIG. 2 is a block diagram of a controller/electromagnetic (EM) circuitryemployed to condition the EM signals provided to the probe and toanalyze reflected EM signals received by the probe.

DETAILED DESCRIPTION

FIG. 1 is a schematic of a tip clearance measurement system employed ona turbine system. In particular, FIG. 1 illustrates rotor 10, having aplurality of blades 12 extending toward stationary case 14. It isbeneficial for engine efficiency for the distance between the tips ofblades 12 and stationary case 14 to be minimized. The distance ‘d’ canbe controlled, but to control properly requires feedback regarding thedistance between blade tips 12 and stationary case 14. This informationis provided by the tip clearance measuring system. The tip clearancemeasurement system may be utilized in a variety of turbine application,such as gas turbine engines, steam turbines, or other well-knownapplications utilizing a turbine assembly.

The tip clearance measurement system includes probe 16 andcontroller/radio frequency (RF) circuitry 18. Although circuitry 18 isdescribed as RF circuitry 18, the frequency of signals provided bycircuitry 18 are not limited to the radio frequency range. For example,the signals may be in the microwave range (e.g., microwave signals). Theterm ‘RF’ is used to generically describe circuitry capable ofgenerating and analyzing signals defined by a characteristic frequency.

Probe 16 is positioned within stationary case 14, and is oriented todirect signals (e.g., microwave or radio frequency) towards turbineblades 12. In addition, probe 16 receives reflected signals.Controller/RF circuitry 18 generates the signals that are provided toprobe 16 and processes the reflected signals sensed by probe 16. Coaxialcable 20 communicates signals bi-directionally between probe 16 andcontroller/RF circuitry 18, including the signals to be directed towardsturbine blades 12 and the reflected signals received by probe 16. Abenefit of the present invention is that only a single coaxial cable isrequired to communicate signals being provided to probe 16 and reflectedsignals received from probe 16. This provides a significant weightsavings over traditional systems. In applications, such as aerospaceapplications, weight is a significant factor in operational cost.Controller/RF circuitry 18 provides an output via output line 22 thatrepresents conditioned/filtered signals received from probe 16 that canbe used to calculate the distance ‘d’ between the turbine blade 12 andstationary case 14.

In one embodiment, the distance ‘d’ is measured by applying a microwavesignal at a first frequency and measuring the resulting signalreflection. Subsequently, another microwave signal is applied at asecond frequency and the resulting signal reflection is measured. Themeasured signal reflections are compared to one another to determine thedistance ‘d’ between turbine blades 12 and stationary case 14. One ofthe microwave signals is designated the reference frequency, and isselected based on the RF circuitry 18, the length of coaxial cable 20and the position of probe 16 (which together can be described as thewaveguide) such that the signal terminates at surface 24 of probe 16. Asa result of the waveform terminating at surface 24, most of the signalassociated with the reference frequency is reflected back from surface24.

At another time, either before or after the reference frequency isapplied, a main frequency is provided by controller/RF circuitry 18 toprobe 16. The main frequency is provided to probe 16 via the samecoaxial cable 20, such that the waveguide (defined by RF circuitry 18,coaxial cable 20, and probe 16) provided for the main frequency isidentical to the waveguide provided for the reference frequency.However, the frequency of the main frequency is selected so that it isnot reflected (substantially) at surface 24 of probe 16, but rather, isdirected toward turbine blades 12. The main frequency is reflected byturbine blade 12, and the reflected signal is received by probe 16. Toallow measurement of the distance ‘d’, the wavelength of the mainfrequency must be at least twice as long as the distance ‘d’. If thedistance ‘d’ is longer than the wavelength, then it cannot be determinedfrom the reflection if the distance is a multiple of the wavelength.

The distance ‘d’ is thus measured by applying the main frequency andmonitoring the resulting reflection. Subsequently, the referencefrequency is applied and the resulting reflection is monitored. Themonitored reflected signal provides an indication of distance traveled.Knowledge of location of surface 24 of probe 16, combined with acomparison of the reflected signals allows the distance ‘d’ to bedetermined. A benefit of the present invention is that because the mainfrequency and the reference frequency are applied at different times,the same waveguide (e.g., RF circuitry 18, the length of coaxial cable20 and the position of probe 16) can be used for both signals. Asdescribed above, a benefit of this approach is that a single coaxialcable can be used to carry both the reference frequency and the mainfrequency. This provides significant weight savings over typical systemsin which at least two coaxial cables are required to simultaneouslycommunicate the reference frequency and the main frequency to the probeand receive reflected signals back from the probe. In addition,utilizing the same waveguide automatically removes errors that wouldotherwise exist between a pair of waveguides.

FIG. 2 is a block diagram of controller/radio frequency (RF) circuitry18 employed to generate frequency signals provided to the probe and toanalyze reflected signals received by the probe. In particular,controller/RF circuitry 18 includes controller 30, first RF conditioningcircuitry 32, second RF conditioning circuit 34, main frequencygenerator 36, reference frequency generator 38, and switch device 40.First RF conditioning circuitry 32 includes multiplier circuit 42,filter circuit 44, splitter circuit 46, and gain circuit 48. Second RFconditioning circuit 34 includes ninety-degree splitter circuit 50,mixer circuit 52, and gain circuit 54.

To generate a signal at the main frequency, controller 30 controlsswitch device 40 to apply main excitation frequency 36 to first RFconditioning circuitry 32. Multiplier circuit 42 multiplies the mainexcitation frequency signal by an integer value (e.g., four). Themultiplied signal is filtered by filter circuit 44, which in oneembodiment is a band-pass filter designed to pass a particular range offrequencies, and filter all others. Splitter circuit 46 splits thefiltered signal into a first and second signal. In the embodiment shownin FIG. 2, splitter circuit 46 is a zero degree splitter, which resultsin no phase difference between the split signals (i.e., split signalsare substantially identical). One of the split signals is provided tosecond conditioning circuit 34 for use in retrieving the reflectedsignals. The other split signal is provided to gain circuit 48, whichamplifies the signal and provides the conditioned main frequency signalto probe 16 (shown in FIG. 1).

To generate a signal at the reference frequency, controller 30 controlsswitch device 40 to apply reference excitation frequency 38 to first RFconditioning circuitry 32. The reference frequency 38 is conditioned inthe same way, by the same components as main frequency 36. A benefit ofthe embodiment shown in FIG. 2, is both the main frequency and thereference frequency are conditioned by the same circuitry, rather thanseparate circuits that each contribute variations that lead to errors.In addition, utilizing the same circuitry reduces the weight, cost andpower associated with RF circuitry.

Reflected signals received from probe 16 are provided to second RFconditioning circuit 34. Once again, both the reflected main frequencyand reflected reference frequency are processed by the same circuitry(i.e., second conditioning circuitry 34). Gain circuit 54 amplifiesreflected signals received from probe 54. The amplified signals are thenprovided to mixer circuit 52. In addition, second RF circuitry 34includes ninety-degree splitter circuit 50, which splits the microwavesignal provided by splitter circuit 46 into sine and cosine components(e.g., separate in phase by ninety degrees). The sine and cosinecomponents provided by splitter circuit 46 represent the frequency andphase of the microwave signal (either reference frequency or mainfrequency) provided to probe 16. The sine and cosine components areprovided to mixer circuit 52, which compares the reflected signal toboth the sine and cosine component to determine the phase of thereflected signal. The resultant outputs of mixer circuit 52 (e.g.,IMain/Ref and QMain/Ref) indicate how closely aligned or in-phase thereflected signal is with the signal provided to probe 16. In theembodiment shown in FIG. 2, the phase of the reflected signal isprovided in the I-Q rotating reference frame. In other embodiments, theoutput of mixer circuit 52 may be converted or expressed in otherwell-understood reference frames.

For example, if a reference frequency is applied to probe 16, and theresulting reference frequency reflection is largely in-phase with thereference signal, then the IRef signal will be greater than the QRefsignal. Likewise, if a main frequency is applied to probe 16, and theresulting main frequency reflection is largely in-phase with the mainsignal, then the Imain signal will be greater than the Qmain signal.Once again, a benefit of the embodiment shown in FIG. 2, is both themain frequency reflections and the reference frequency reflections areconditioned by the same circuitry, rather than separate circuits thateach contribute variations that lead to errors. In addition, utilizingthe same circuitry reduces the weight, cost and power associated with RFcircuitry.

In one embodiment, the outputs generated by second RF circuitry 34(e.g., IMain/Ref and QMain/Ref) are provided to controller 30 forstorage and subsequent processing to determine distance ‘d’. In otherembodiments, these outputs may be provided to another processor (notshown) for storage and processing to determine distance ‘d’. Becauseonly one of the reference frequency or main frequency is applied at atime, the monitored reflections must be stored. Once both the referencefrequency and main frequency have been applied and reflections monitored(in either order), then a comparison of the monitored reflections isused to determine the distance ‘d’.

In operation then, controller 30 would at a first time control switchdevice 40 to apply the main frequency to probe 16 via first RFconditioning circuitry 32. A reflected main frequency received fromprobe 16 would be analyzed by second RF conditioning circuitry 32 toprovide a phase comparison between the main frequency and reflected mainfrequency. At a subsequent time determined by controller 30, switchdevice 40 is controlled to apply the reference frequency to probe 16 viafirst RF conditioning circuitry 32. A reflected reference frequencyreceived from probe 16 would be analyzed by second RF conditioningcircuitry 32 to provide a phase comparison between the referencefrequency and the reflected reference frequency. Based on the resultingphase comparisons associated with the main frequency and the referencefrequency, a distance ‘d’ can be calculated either by controller 30 oranother processing device.

In this way, the present invention provides a tip clearance measurementsystem that avoids redundant circuitry to condition and analyze RFsignals used to determine tip clearance. In addition, utilizing the samecircuitry for both the reference frequency signal and the main frequencysignal prevents differences or variations in the redundant circuitryfrom affecting the monitored reflections.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A tip clearance measurement system (TCMS) according to an exemplaryembodiment of this disclosure, among other possible things includes aprobe and radio frequency (RF) circuitry. The probe both directs signalstoward a turbine blade and receives signals reflected by the turbineblade. The RF circuitry includes a switch having a first state in whicha main frequency is provided at an output of the switch and a secondstate in which a reference frequency is provided at an output of theswitch. The RF circuitry further includes a first conditioning circuitthat receives a frequency provided at the output of the switch andprovides a conditioned frequency to the probe and a second conditioningcircuit that receives both the conditioned frequency provided by thefirst conditioning circuit and a reflected frequency received by theprobe, and provides a conditioned output based on the conditionedfrequency and reflected frequency.

The TCMS of the preceding paragraph can optionally include, additionallyand/or alternatively, any one or more of the following features,configurations and/or additional components:

A further embodiment of the foregoing TCMS, which may further include acoaxial cable connected between the probe and the sensing circuitry.

A further embodiment of the foregoing TCMS, which may further include acontroller that selectively controls the state of the switch to applyeither the main frequency or the reference frequency to the output ofthe switch.

A further embodiment of the foregoing TCMS, wherein the controller mayapply the main frequency to the output of the switch, and thensubsequently apply the reference frequency to the output of the switch.

A further embodiment of the foregoing TCMS, wherein the conditionedoutput provided by the second conditioning circuit may be provided inthe I-Q reference frame.

A further embodiment of the foregoing TCMS, wherein the controller maystore the conditioned output provided by the second conditioning circuitfor both the main and reference frequency reflections.

A further embodiment of the foregoing TCMS, wherein the controller maydetermine tip clearance based on the stored main and reference frequencyreflections.

A method of measuring tip clearance in a turbine includes controlling aswitch to a first state to apply a main frequency signal to a probe viaa first conditioning circuit, wherein the first conditioning circuitprovides a first conditioned signal to a probe. The method furtherincludes maintaining the switch in the first state while a firstreflected signal is received by the probe and conditioned by a secondconditioning circuit, wherein the second conditioning circuit conditionsthe first reflected signal based on the first conditioned signal toprovide a first output. The method further includes controlling theswitch to a second state to apply a reference frequency signal to thefirst conditioning circuit, wherein the first conditioning circuitprovides a second conditioned signal to the probe. The method furtherincludes maintaining the switch in the second state while a secondreflected signal is received by the probe and conditioned by the secondconditioning circuit, wherein the second conditioning circuit conditionsthe second reflected signal based on the second conditioned signal toprovide a second output. Tip clearance is calculated based on the firstoutput and the second output.

A further embodiment of the foregoing method, which may further includecontrolling an amount of time between controlling the switch to thefirst state and controlling the switch to the second state.

A turbine system according to an exemplary embodiment of thisdisclosure, among other possible things includes a plurality of turbineblades extending radially outward from a rotor, a stationary casesurrounding the plurality of turbine blades, and a tip clearancemeasurement system (TCMS). The TCMS system includes a probe locatedwithin the stationary case and oriented to direct signals toward theplurality of turbine blades and receive signals reflected by theplurality of turbine blades. The TCMS system further includes RFcircuitry that includes a switch having a first state in which a mainfrequency is provided at an output of the switch and a second state inwhich a reference frequency is provided at an output of the switch, afirst conditioning circuit that receives a frequency provided at theoutput of the switch and provides a conditioned frequency to the probe;and a second conditioning circuit that receives both the conditionedfrequency provided by the first conditioning circuit and a reflectedfrequency received by the probe, and provides a conditioned output basedon the conditioned frequency and reflected frequency.

A further embodiment of any of the foregoing turbine systems, which mayfurther include a coaxial cable connected between the probe and thesensing circuitry.

A further embodiment of any of the foregoing turbine systems, which mayfurther include a controller that selectively controls the state of theswitch to apply either the main frequency or the reference frequency tothe output of the switch.

A further embodiment of any of the foregoing turbines, wherein thecontroller may apply the main frequency to the output of the switch, andthen subsequently apply the reference frequency to the output of theswitch.

A further embodiment of any of the foregoing turbines, wherein thecontroller may store the conditioned output provided by the secondconditioning circuit for both the main and reference frequencyreflections

A further embodiment of any of the foregoing turbines, wherein thecontroller determines tip clearance based on the stored conditionedoutputs provided by the second conditioning circuit.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A method of measuring tip clearance in aturbine, the method comprising: controlling a switch to a first state toapply a main frequency signal to a probe via a first conditioningcircuit, wherein the first conditioning circuit provides a firstconditioned signal to a probe; maintaining the switch in the first statewhile a first reflected signal is received by the probe and conditionedby a second conditioning circuit, wherein the second conditioningcircuit conditions the first reflected signal based on the firstconditioned signal to provide a first output; controlling the switch toa second state to apply a reference frequency signal to the firstconditioning circuit, wherein the first conditioning circuit provides asecond conditioned signal to the probe; maintaining the switch in thesecond state while a second reflected signal is received by the probeand conditioned by the second conditioning circuit, wherein the secondconditioning circuit conditions the second reflected signal based on thesecond conditioned signal to provide a second output; and calculatingtip clearance based on the first output and the second output.
 2. Themethod of claim 1, further comprising: controlling an amount of timebetween controlling the switch to the first state and controlling theswitch to the second state.
 3. The method of claim 1, wherein the firstconditioning circuit provides both the first and second conditionedsignals to the probe via a coaxial cable.
 4. The method of claim 1,wherein controlling the switch to the first state, maintaining theswitch in the first state, controlling the switch to a second state, andmaintaining the switch in the second state are performed by a controllerthat selectively controls the switch.
 5. The method of claim 1, whereincontrolling a switch to a first state to apply a main frequency signalto a probe is performed prior to controlling the switch to a secondstate to apply a reference frequency signal to the first conditioningcircuit.
 6. The method of claim 1, wherein the second conditioningcircuit conditions the first reflected signal based on the firstconditioned signal to provide the first output in an I-Q referenceframe.
 7. The method of claim 1, further comprising: storing the firstoutput and the second output.
 8. The method of claim 1, whereincalculating the tip clearance based on the first output and the secondoutput comprises comparing the first output and the second output todetermine the tip clearance.